Improve test coverage of bignum operation functions

[gilles-peskine-arm]

Follow-up of #5959. The goal here is to make the coverage of edge cases more systematic.

For each bignum input argument to a test function, make sure that:

• There is at least one test case where the argument is positive with a single limb (<2^32), one where it's positive with multiple limbs (≥2^64), one where it's negative with a single limb, one where it's negative with multiple limbs, one where it's negative and one where it's zero.
• Whenever testing a bignum input, also test that input with a leading zero limb.
• When there are multiple inputs, test interesting cases, e.g. equal vs ≥ vs ≤ values for subtraction.

Remove manually written test data that's no longer relevant.